1. Field of the Invention
The present disclosure relates to an optical semiconductor and a method for producing the optical semiconductor, and to an optical semiconductor device, a photocatalyst, a hydrogen producing device that includes the photocatalyst, and an energy system that includes the hydrogen producing device.
2. Description of Related Art
In an optical semiconductor, electron-hole pairs are generated by irradiation of the optical semiconductor with light. Optical semiconductors are promising since they can be used for applications such as LEDs and lasers for extracting light generated in recombination of the electron-hole pairs, solar cells for extracting the photovoltaic power as electrical energy by separating the pairs spatially, and photocatalysts for producing hydrogen directly from water and sunlight. As one group of optical semiconductors that absorb or emit light in the ultraviolet to visible light region, nitrides and oxynitrides can be mentioned. Examples thereof include nitrides and oxynitrides of tantalum, titanium, aluminium, gallium, indium, etc. However, no reports have been made so far on the use of niobium oxynitride as an optical semiconductor. Reported methods for synthesizing niobium oxynitride and the physical characteristics thereof are shown below.
As a method for producing niobium oxynitride, a method of using niobium oxychloride, etc., as a raw material and a method of calcinating niobium pentoxide in an ammonia atmosphere, for example, are known (see, for example, Zeitschrift für anorganische und allgemeine Chemie, 429, 261-269 (1977), Journal of The Electrochemical Society, 156 (7) B811-B815 (2009), and JP 2009-208070 A).
Zeitschrift für anorganische und allgemeine Chemie, 429, 261-269 (1977) discloses niobium oxynitride synthesized using niobium oxychloride or niobium pentachloride as a niobium source, and the synthesis method thereof.
In the case of using niobium oxychloride as a starting material, niobium oxychloride (NbOCl3) and liquid ammonia are reacted with each other. In the sample obtained by the reaction, ammonium chloride (NH4Cl) is present in a mixed state as a by-product. Therefore, in order to remove the ammonium chloride from the material through sublimation, the obtained material is heated at 653 K under vacuum. The sample after the heating is further calcinated at 773 K for a long period of time. This allows niobium oxynitride (NbON) that is black in color to be obtained.
In the case of using niobium pentachloride as a starting material, niobium pentachloride (NbCl5) and nitrogen chloride (ClN3) are first reacted with each other to obtain niobium tetrachloride azide (NbCl4N3). The resultant niobium tetrachloride azide is hydrolyzed to form niobium oxychloride azide (NbOCl2N3). This is subjected to pyrolysis at 773 K, thereby allowing niobium oxynitride (NbON) to be obtained.
Zeitschrift für anorganische and allgemeine Chemie, 429, 261-269 (1977) discloses that niobium oxynitride obtained by the above-mentioned method has a crystal structure of baddeleyite and is represented by the composition formula of NbON, in accordance with the X-ray crystal structure analysis.
Journal of The Electrochemical Society, 156 (7) B811-B815 (2009) and JP 2009-208070 A each disclose niobium oxynitride synthesized using niobium pentachloride as a niobium source by heat treatment in a flowing ammonia gas atmosphere, and the synthesis method thereof. Niobium chloride (NbCl5) is employed as a raw material. This is dissolved by stirring in a solution consisting of ethylene glycol (HOCH2CH2OH), citric acid (HOOCCH2C(OH)(COOH)CH2COOH), and methanol (CH3OH). Thereafter, this solution is stirred in an air atmosphere while being heated to 423 K. This causes dehydration esterification reaction between the carboxyl group of oxycarboxylic acid and the hydroxyl group of glycol, thus allowing a polyester polymer gel that has undergone polyesterification to be obtained. Then, the resultant gel is subjected to heat treatment at 773 K for 3 hours in a flowing nitrogen atmosphere at 0.1 L/minute. Subsequently, it is further subjected to heat treatment at 1023 K for 10 hours in a flowing ammonia gas atmosphere at 0.5 L/minute. Thus, niobium oxynitride (Nb—O—N) is obtained. The X-ray diffraction pattern disclosed in Journal of The Electrochemical Society, 156 (7) B811-B815 (2009) shows that this Nb—O—N is a mixture of niobium pentoxide (Nb2O5) and niobium nitride (NbN).